In recent years, copying machines and printers using an electrophotographic or ink jet recording method have undergone remarkable improvements in performance. Specifically, they have made spectacular progress in not only copying and printing speeds but also image qualities, particularly in full-color copying and printing arts. However, recording apparatus, such as copying machines and printers of the aforementioned type, cannot fully achieve their excellent performances unless they match well with recording paper used therein.
In accordance with an ink jet recording method, the recording is generally carried out using water base ink, and so it has drawbacks of being inferior in drying speed of ink and suffering from ripple generation on the recording paper surface after printing.
For the recording paper applied to an ink jet recording system, therefore, it is required to have not only properties of ensuring high-speed drying of ink, providing recorded images of high optical density and causing neither overflowing nor feathering of ink but also a property of causing no ripple on the recording paper after absorption of ink. The generation of ripple and curl in an ink jet recording paper vary in their behaviors according to the ink absorption characteristics which the recording paper has, the surface tension and the permeability of ink used for the recording, the size of ink drops jetted, the amount of ink driven into the recording paper, the recorded area and so on. Further, their behaviors just after printing differ from those after spontaneous drying of the printed ink.
In order to reduce rippling and curling troubles occurring just after printing, various arts are disclosed. As for the ripples caused on the so-called pigment-coated ink jet recording paper, for instance, the method of preventing the ripple generation by the use of a raw paper having excellent dimensional stability is disclosed in Japanese Tokkai Sho 62-95285 (The term "Tokkai" as used herein means an "unexamined published patent application"). Further, Japanese Tokkai Hei 6-171206 pays special attention to a moisture change in the recording paper, and discloses the method of giving a particular treatment to the back side of a recording paper. Since the rippling and curling troubles occur due to expansion of paper upon recording, the point aimed at by both of those methods consists in securing the dimensional stability of a recording paper. In other words, the arts cited above intend the ripple generation problem to be solved by using a dimensionally stable paper and by giving a dimensional stability improving treatment to a recording paper, respectively.
As for the curl and ripple generation after spontaneous drying of printed ink, on the other hand, ink jet recording papers have not yet had substantial improvements therein. In a case where the undulation and the curling of a recording paper are smoothened out by half compulsion, e.g., with a decurler or the like, as disclosed in Japanese Tokkai Hei 7-186519, such distortion of the recording paper disappears seemingly, but the distortion still remains inside the recording paper. Accordingly, the distortion emerges into the paper surface when the recording paper is allowed to stand in such an environment as to change the moisture therein. Thus, such a half compulsory smoothening cannot dissolve the distortion in a substantial sense.
There are many reports on the curl of a general recording paper. In particular, the reversible curl have been analyzed in detail (e.g., in Kami Pa Gikyo Shi, Vol. 39, No. 10; ibid., Vol. 41, No. 4; ibid. , Vol. 43, No. 7). The reversible curl signifies the curl appearing in relatively high reproductivity when the environment of a recording paper is repeatedly changed in humidity, and the theoretical analysis thereof has been well in progress. As a result thereof, for instance, it has turned out that the so-called heat curl of an electrophotographic transfer paper, namely the curl after thermal fixation, can be controlled by restricting a paper's shrinkage factor to a specified range, wherein the shrinkage factor is defined as a shrinkage factor which the paper has on the last dehumidifying cycle when it undergoes three cycles of specified changes in environmental humidity to make reversible expansion and shrinkage cycles (Japanese Tokkai Hei 5-34155).
The irreversible curl, on the other hand, generates during the first cycle of changes in humidity, and the curl generating in the course of humidity change to the higher side is irreversible in particular. Far from being well analyzed, it is the present condition that the irreversible curl is hardly recognized yet.